1. Field of the Invention
This invention relates to a method for generating a pulse train having a variable frequency. Particularly, this invention relates to a method for generating a pulse train in which the output frequency is generated by division of a reference frequency of a predetermined period by an adjustable division factor. This invention also relates to a circuit arrangement for carrying out the method. In particular, this invention relates to the generation of ignition pulses for a converter.
2. Description of the Prior Art
For example, from the German book "Elektronische Bausteinsysteme SIMATIC" by Rolf Hahn, volume 1, 1974, pages 244 to 245, a frequency divider is known in which a reference frequency is multiplied by a factor between 0 and 0 in order to obtain a desired output frequency. The multiplication method described in this publication may be employed also for the generation of a pulse train having an adjustable or variable frequency if one uses as reference frequency f.sub.O a fixed frequency which is generated, for example, by a quartz oscillator with high accuracy. The fixed frequency f.sub.O may be divided by a programmable divider by an adjustable division factor p. At the divider output, a pulse train having the frequency f.sub.A =f.sub.O/p will be obtained. With such a method, however, the frequency resolution .DELTA.f, i.e. the smallest possible frequency variation, will become worse with the square of the output frequency f.sub.A.
For p&gt;&gt;1, there applies with very good approximation: EQU .DELTA.f=f.sub.A.sup.2 /f.sub.O =1/f.sub.O .multidot.T.sub.A.sup.2, with T.sub.A =1/f.sub.A.
This equation demonstrates that the absolute frequency resolution .DELTA.f becomes worse with the square of the output frequency f.sub.A. It is also obvious from this equation that the relative frequency resolution .DELTA.f/f.sub.A will become worse proportionally to the output frequency f.sub.A. For instance, if the required maximum output frequency f.sub.Amax is 2400 Hz and the oscillator frequency f.sub.O is 2 MHz, a frequency resolution .DELTA.f=2400.sup.2 Hz/2.multidot.10.sup.6 =2.88 Hz will be obtained. For many applications, for example in frequency generators for converters, much better frequency resolutions, e.g. better than 0.6 Hz, are required.
An improvement of the frequency resolution might be obtained by increasing the oscillator frequency f.sub.O. This, however, is often not possible for technological reasons, e.g. because of the limited working frequency of the components used.